Bile acid studies were performed in patients with Crohn's disease, radiologically confined to the colon. The bile acid pool size of 10 patients with isolated Crohn's colitis was significantly lower than that of 10 normal control subjects (P less than 0.001) and of 10 ulcerative colitis patients (P less than 0.005). Measurements of 14C-excretion in breath and in 24 hours stool collections after the administration of 5 muCi 14C-glycocholate showed a normal 14C-excretion in breath and usually a markedly increased loss of 14C in the stool (greater than 7% of the dose). The simultaneous administration of 5 muCi 3H-polyethylene glycol MW 4000 (3H-PEG) as a marker indicated that the 14C/3H ratio in the patients with Crohn's colitis was significantly greater than in a control series of patients with diarrhoea not due to bile acid malabsorption. Studies on the composition of duodenal bile showed a significantly decreased concentration of deoxycholic acid in duodenal bile. These observations suggest bile acid malabsorption in patients with Crohn's disease apparently confined to the colon.